The present invention relates generally to methods for monitoring and neutralizing contaminants in a fluid stream. More specifically, the present invention relates to methods and apparatuses for monitoring and/or neutralizing chloride levels in fluid streams in oil refineries.
During the refinery process, crude oil introduced into distillation columns contains water, organic acids and chemical precursors to various strongly acidic species. Due to salts comprised of chloride that are cracked in the distillation process, chloride is present in such fluid streams.
Determining the concentration of the chloride, and neutralizing same, is critical because the chloride can form hydrochloric acid. Of course, hydrochloric acid is extremely corrosive and if not neutralized can damage piping and vessels that contain such fluids. Typically, hydrochloric acid is one of many acidic species that are generated in fluid streams at the distillation tower of petroleum refineries.
Various methods have been suggested to remedy acidic corrosion. Generally, a neutralizing agent is introduced, either directly to the crude charge and/or injected at the top of the distillation tower. The quantity of neutralizing agent to be added is usually determined following some manual sampling and acid analysis of the tower condensate or of some other species. Because the time elapse between these manual analyses may span one or more days, excursions in the acid concentrations of the overhead water may occur. Because of the rapidity of acid corrosion, even a one day's unmonitored and uncontrolled excursion can have serious consequences.
U.S. patent application Ser. No. 07/867,890, filed Apr. 13, 1992, entitled: "ON-LINE ACID MONITOR AND NEUTRALIZER FEED CONTROL OF THE OVERHEAD WATER IN OIL REFINERIES" discloses an apparatus and method for monitoring and controlling the concentration of acid in the overhead water of oil refineries. Although this system works very well at controlling acid concentration, it is desirable in at least some situations to have a system for also monitoring only chloride concentration and allowing the system disclosed in the above-identified patent application to measure and control other acid species.
It is presently very difficult to continuously measure the concentration of chloride in such fluid. For example, although chloride specific electrodes exist, these cannot typically be used efficiently in a fluid stream from a refinery. This is due to the fact that sulfide, which may also be present in the fluid stream, will interfere with the chloride specific electrode's measurement of chloride. This will result in the reporting of incorrect chloride concentrations.
Although attempts at resolving this problem have been tried, for example, U.S. Pat. No. 4,942,133, these attempts are not entirely satisfactory. For example, as disclosed in the U.S. Pat. No. 4,942,133, in order to analyze chloride ion content, the following procedure is utilized. Drawing from the overhead waters, a stream of water for analysis; removing from the sample so drawn, the hydrocarbons as well as the suspended matter; blowing nitrogen into the sample to sweep away the hydrogen sulfide and the residue hydrocarbons; oxidizing the S.sup.2- or HS.sup.- sulfide ions to sulfate ions; acidifying the sample; and finally measuring by a combined electrode, the content of chloride ions present. To the best of the inventor's knowledge, this system has not been commercialized, at least on a large scale.
There is therefore a need for a method for the automatic, on-line monitoring of chloride in fluid streams, especially in oil refineries, for the purpose of controlling corrosion.